While not needing to do exactly what you are doing I am using the Arduino-SerialCommand library that was originally devised by Steven Cogswell and modified by Stefan Rado to send commands with arguments to an Arduino from Processing. I see that SerialCommand was initially created for stepper motor control so perhaps it is suited for what you are doing. On the Adruino side you can control the SerialCommand input buffer size. So I would think you could make sure your Processing side data sender sends data within that buffer size. This SerialCommand library fills its input buffer with incoming data while waiting for an incoming \n to signal the end of the command stream. It then looks at the first word in that command stream to assoicate with callback routines that you create to process whatever is in the input buffer. Maybe that communication scheme will work for you.    

I ended up solving it with the following:

OK. Are these motor control commands executed in realtime or are they first stored and then executed? I am wondering if the pauses cause any issues. By the way, does your Processing application ride through a temporary serial disconnection from the Adruino and if so how is that accomplished? 

They're stored in the serial buffer, which can store up to 64 bytes. To be on the safe side, I only send 32 bytes at a time (waiting==32). As soon as the arduino has processed those 32 bytes (which can be any of A through H, each corresponding to a direction of travel), it sends the command to resume to processing, which sends the next 32 bytes. In practice, there's no noticeable delay - the motors are in constant movement.